The present invention generally relates to heating control devices for thermal ink-transfer type printing apparatuses (hereinafter referred to as thermal printing apparatuses), and more particularly to a heating control device for a thermal printing apparatus, in which heating is controlled so that currents are not simultaneously applied to mutually adjacent heating resistors among heating resistors of a plurality of head elements constituting a single thermal printing head.
Among terminal printers or hard-copy apparatuses such as wire-dot type, shuttle type, and ink-jet type printers, thermal printing apparatuses are being developed as one of the more promising type. For example, this thermal printing apparatus employs an ink film which is a polyester film having a thickness of 5 to 6 .mu.m coated with a kind of ink which melts due to heat on one surface thereof. The ink film is placed onto a recording sheet with the ink side making contact with the recording sheet, and a thermal printing head makes contact with a rear side of the ink film. When a current flows through the thermal printing head so as to generate heat at the printing head, the ink on the ink film melts at the position corresponding to the position of the printing head, and the melted ink is transferred onto the recording sheet. This thermal printing head comprises a plurality of head elements arranged in a row, and a current is successively applied to each of these head elements.
The density which determines the tone of the printed characters, figures, diagrams, and the like, is determined by the area of each dot formed on the recording sheet due to the transfer of the melted ink onto the recording sheet. And, this area of the melted ink dot is determined according to the heating temperature of each of the head elements. Generally, the heat value becomes larger as the magnitude of the currents applied to the head elements become larger, or as the duration with which the currents are applied to the head elements becomes longer. As a result, the area of the melted ink dot becomes larger to increase the printing density, and the tone reaches near a saturated density.
However, the distance between centers of adjacent heating resistors constituting each of the head elements is 125 .mu.m, for example, and the spacial distance between adjacent heating resistors is 20 .mu.m, for example, and the heating resistors are provided exceedingly close to each other. Hence, if an attempt is made to increase the area of the melted ink dot so as to increase the printing density, that is, if an attempt is made to increase the heating temperature of each of the heating resistors, the temperature rises at intermediate points between the adjacent heat resistors due to the heat exerted by the adjacent heating resistors. Accordingly, although the dots are to be formed for each of the heating resistors, the ink also melts at the intermediate points between adjacent heating resistors, and led to a phenomenon in which the dots become connected. In this case, the printed characters, figures, and the like became unclear, and there was a disadvantage in that the printing could not be carried out accurately by the dots. On the other hand, if the heating temperature is set to a low temperature so as not to introduce the above undesirable phenomenon, the area of each of the dots become exceedingly small, and the maximum saturated density becomes low.